It is today's (August 14) Super Hard at brainbashers.com. I didn't post it because it didn't help solve the puzzle, and I finally found an xy-wing that brought the puzzle down all at once, but here it is:

It seems like it wouldn't be that rare to find that pattern in two boxes with trips remaining, but I certainly never looked for it before. I suppos some would argue that it is like forcing chains in a UR format.

I did note the potential URs, but the xy-wing jumped out at me as I was looking at them and it solved the puzzle.

If only the puzzle didn't have the xy-wing solve -- it would be unique in my (limited) experience, and my solve on the R37C78 would have in fact been criotical. (I get discouraged when I think I've found a strong solve, and it turns out there was a simpler way to do it.)

I realize now that uniqueness theory has always been critical to my solving techniques for seriously difficult puzzles. It is that imbalance, the lack of symetry, the sense that a certain group of numbers just can't quite fit into the allotted space, that points the way -- almost invariably -- to a significant and helpful solution.